Process for producing microperforated stainless steel sheets



Nov. 14, 1967 3,352,769

S. RUBEN PROCESS FOR PRODUCING MICROPERFORATED STAINLESS ST SHEETS FiledJan. 1967 INVENTOR. SAMUEL RUBEN United States Patent 3,352,769 PROCESSFOR PRODUCING MICROPERFORATED @TAINLESS STEEL SHEETS Samuei Ruben, 52Seacord Road, New Rochelle, 10804 Filed Jan. 11, 1967, Ser. No. 615,28813 Claims. (Cl. 204-143) ABSTRACT 9F THE DHSCLUSURE The invention is aprocess for producing microperforated stainless steel sheets. Itcomprises connecting a sheet of stainless steel as the anode in anelectrolytic cell containing a non-polarizing electrolyte anddischarging direct current through the cell to produce a multiplicity oflight transmitting microperforations through the sheet, the perforationsbeing due primarily to electrochemical action.

This application is a continuation-in-part of my copending applications:Ser. No. 422,005 filed Dec. 29, 1964, which was a continuation-in-partof my application Ser. No. 345,071 filed Feb. 17, 1964; Ser. No. 472,617led July 16, 1965, which was a continuation-in-part of application Ser.No. 304,667 filed Aug. 26, 1963; Ser. No. 556,921 filed June 13, 1966,and Ser. No. 585,054 filed Oct. 7, 1966, all of which have becomeabandoned.

This invention relates to -a process for producing microperforatedstainless steel sheets and to products made therefrom such as filtersand diaphragms. In a preferred form, microperforated stainless steelstrip is produced in a continuous operation.

An object of the invention is to provide a simple, inexpensive processfor producing microperforated stainless steel.

A further object is to provide a microperforated stainless steel sheetadaptable for use as filter, diaphragm and separator elements.

Other objects will be apparent from the disclosure and the drawing whichillustrates the process in schematic form.

Various means have previously been used for producing metal platefilters, for instance, by pressing and sintering metal powder to producea porous mass. In the present invention, relatively thin non-porousstainless steel sheet produced from east and rolled alloy is utilized asthe starting material.

The invention comprises connecting a sheet of stainless steel as theanode in an electrolytic cell containing a nony polarizing electrolyteand discharging direct current through the cell to produce amultiplicity of light transmitting microperforations through the sheet,the perforations being primarily due to electrochemical action.

A method is provided for uniformly perforating thin stainless steelsheet in a continuous process whereby microperforations are obtained atthe grain boundary of the metal crystal structure with selective anodicdissolution at these points and with negligible anode dissolution of theface of the crystals so that the thickness of the sheet remainssubstantially the same. The microperforations are substantiallyuniformly distributed throughout the sheet and vary with the compositionof the alloy.

The process utilizes stainless steels which are formed by the additionof chromium to iron, with a minute quantity of carbon and containing alesser amount of nickel, such as 8%, as well as stainless steelscomposed essentially of iron and chromium without the addition ofnickel, the chromium usually being present in the order of 12% or more.Both types of stainless steel may contain minor additions of othermetals. All of the various types and grades 3,352,769 Patented Nov. 14:,1967 of stainless steel, as commonly known, including the #200, #300,#400 and #500 series may be utilized. Stainless steels generally containa minimum of approximately 11 /2 chromium. An essential factor forproducing uniform microporosity is the iron content, as without iron,anodic grain boundary perforations would not occur, thesemicroperforations being obtained without substantially affecting eitherthe thickness or the smooth light reflecting surface of the stainlesssteel sheet.

While hydrochloric acid of a concentration in the order of 2% and notexceeding 7% is the preferred electrolyte, other non-polarizingelectrolytes may be employed, for example, hydrobromic acid and halogensalts which when dissolved, hydrolize to an acidic electrolyte, such asthe chlorides and bromides of iron, nickel, cobalt or manganese. Thealkali and alkaline earth metal chloride salts and ammonium chloridesalts will not provide anodic perforation of the chromium nickelstainless steels, such as the 300 series, but will uniformly perforatethe nickel free types, such as the 400 series. Unpredictably, I havealso found that the bromides of the alkaline earth metals willanodically perforate both types of stainless steel.

When chromium nickel stainless steel, such as No. 302, containingapproximately 8% nickel is made the anode in an electrolyte of an alkalior alkaline earth metal halogen salt, such as the chlorides of Li, Na,K, Sr, "Be, Mg, Ca and Ba, or the ammonium halogen salt or sea water,only a slight surface etch is noted with an occasional large pit. Theseelectrolytes do not appear suitable for use in the microperforatingprocess. However, when nickel-free stainless steel such as #430 isconnected as the anodein the same type electrolyte, substantiallyuniform microperforations are obtained.

1 have also found that bromides of the alkaline earth metals, forinstance, the bromides of Ca, Mg, Sr and Ba, as well as lowconcentrations of hydrobromic acid such as 2% will provide perforationof both nickel and nickelfree stainless steels. Apparently theunexpected result with the bromide electrolytes is due to the di-valentnature of the basic ion. Current density and time are generally the samefor the bromide electrolytes as the chloride electrolytes.

For operation over long periods it is desirable to ad an adequate amountof either hydrochloric acid or hydrobromic acid to dissolve the oxidesformed during process ing and particularly to prevent the rise in pH ofthe electrolyte. The salt solutions may in some instances be madeinitially acidic through the addition of HCl or HBr to maintain aminimum polarization or anodic voltage drop.

A basic factor for the operation of this process is the non-polarizingcharacter of the electrolytes. The intergranular precipitates aredissolved into the electrolyte and the formation of a passive oxidesurface, usually associated with anodic effect with stainless steel, isavoided.

When hydrochloric acid is used as the electrolyte, the concentrations ofthe acid are held to definite limits. For instance, if No. 430 stainlesssteel is processed at a temperature of 25 C., the concentration of thehydrochloric acid is critical and must be kept below 4.7%. My tests showthat a concentration of 4.65% will not allow perforation of the sheetwhile a concentration of 4.6% hydrochloric acid at a current density of6 amperes for 4 minutes on a 12 inch square area provides a uniformperforation. The #400 series of stainless steels are substantiallynickel free, grade #430 having an approximate of .12% carbon, 1%manganese, 1% silicon, 14% to 18% chromium and the remainder iron. The#300 series of stainless steels contain nickel. For example, type 304contains approximately 8% nickel and approximately 18% chromium, the

' balance being substantially iron. At 25 C., with stainless steelcompositions of these types, the concentration of hydrochloric acid inthe solution must not exceed 7%.

It should be understood that when hydrochloric acid is used, theelectrolyte is only initially pure hydrochloric acid. When the anodictreatment of the stainless steel commences, anodic dissolution of theprecipitated chormum-iron compounds at the grain boundaries takes placeand the color of the electrolyte changes froma clear solution to a greensolution containing chromium and iron chlorides, with the chromiumchloride content apparently being higher than that of the iron chloridecontent. The amount of, chromium or iron chlorides produced depends uponthe length of time the solution is used and the reduction of the initialhydrochloride acid content can be noted by the rise in pH duringprocessing. To maintain uniformity over a period of time duringcontinuous operation of the process, hydrochloric acid is added whenrequired to keep the pH at the desired value. The preferred initialelectrolyte is hydrochloric acid of a concentration of approximately 1%to 2% and as mentioned above, this may be maintained by the addition ofsmall amounts of hydrochloric acid during continuous processingpTheconcentration should desirably not substantially exceed 2%.

Perforations will not occur when stainless steel sheet is immersed inhydrochloric acid. Without the application of current; and withconcentrations of 4.65% or less, there is negligible effect on thethickness and weight of the sheet, even after a long period ofimmersion. With the higher concentrations of hydrochloric acid with orwithout the application of current, etching and dissolving of thesurface of the entire sheet takes place with reduction in thickness andweight.

When ferric chloride is employed as the electrolyte, the concentrationmay vary between 5% to 50%, preferably between and 20%. The pH of theferric chloride electrolyte is importantthe lower the pH the lesscurrent density and time required for a given degree of porosity. Atypical solution having a concentration of 20% FeCl .6H O has a pH ofabout 1.1. The concentrations of the other halogen salt electrolytes maybe of the same order as that of the ferric chloride electrolytes.

While the fluorides are more expensive and more difiicult to handle,some of the soluble fluorides, such as CrF .4H O, may be used.

Referring to the drawing, the roll of .0025 inch thick No. 430 stainlesssteel 1 constituting the anode, is passed through a HCl electrolyte 2via plastic roller 4, the foil being kept under tension and passingthrough the electrolyte at a speed determined by the degree of porositydesired. For example, a 6" x 6 wide area would be immersed in theelectrolyte for 4 minutes with an applied current of 18 amperes or 288ampere minutes persquare foot. The cathode 3 is a strip of titaniummaterial which extends over the area being treated. Titanium exhibits aspecial superiority for this use. It is inert in the electrolyte,possesses adequate conductivity and is capable of continuous use withoutchemical or electro-chemical effect. After passing through theelectrolyte, the stainless steel foil passes into two wash tanks 9 and10 via rollers 5, 6, 7 and 8, the Water 11 and 12 in the tankseliminating any retained electrolyte. A supersonic transducer may beutilized to provide additional cleaning action and to preventundesirable electrode polarization or anodic cleaning may also beemployed. The foil is then passed through rollers 13 and wound into.coil 14, motor 15 serving as the driving means. Filter 16 and pump 17insure a continually circulating filtered electrolyte. If desirable, thespeed and current can be photoelectrically controlled by measuring thelight'transmission through the perforated foil after washing and thedegree of perforation can thereby be controlled Within close limits. Thestainless steel sheet so processed retains a smooth light reflectingsurface and its mechanical strength.

In order to maintain uniformity in a continuous process, it isnecessary, as mentioned above, to replace the halogen content which canbe done by adding an acid component; for instance, with chlorides it isdesirable to add hydrochloric acid to prevent the electrolyte pH fromincreasing too much and with the bromides, hydrobromic acid. The use ofsea water. or other natural brine Water can provide a continuous supplyof electrolyte without the necessity of adding an acid component.

It is desirable to control the temperature of the electrolyte so that itdoes not rise much above 30 C., in order s that the perforationsproduced in the stainless steel Will be uniformly controlled by thecurrent density and time and due only to the electrochemical anodicaction. Control of the temperature will produce a duplicatable porositywith holes of uniform size and shape and with adequate heat dissipationarea of the electrolyte container or by water cooling, this temperaturelimit can be maintained. By control of the density applied to theelectrode and the speed of the stainless steel strip through theelectrolyte, variations in the porosity can be effected through a Widerange.

The nature of the current is of considerable importance, substantiallycontinuous unidirectional current being highly desirable. For example,if alternating current is applied at the preferred current discharge of72 amperes per square foot, there is no perforation above a negligibleamount-less than 0.5 %-and this could be ascribed to solution effect. Ifhalf wave recitified direct current is applied at 72 amperes per squarefoot, a photometer reading of 8 is obtained, compared with a reading of55 where full wave rectified current is utilized. When continuous directcurrent from a battery is applied, a comparative light transmissionvalue of 64 is obtained. With a three.

phase rectified full wave current of the same density, a comparativelighttransmission of 62 is obtained.

The voltage required for operation of the continuous process will varywith temperature and electrolyte concentration, size, spacing and areaof the electrodes. In a continuous process, with current density ofabout 72 amperes per square foot, it may be in the order of 12 volts DC.

The concentration of electrolyte depends to some extent upon the desiredsize of the perforations. For most applications I have found anelectrolyte concentration of 2% with HCl, or HBr, 4% CaBr and 10% withFeCl .6H O, to be satisfactory. The thickness of the stainless steelsheet should preferably be not greater than .005" and not thinner than.0005, a preferred range being from .002" to .003".

The number of perforations may vary considerably with current, time andalloy composition. In general, they may average in the order of 10 to 40microns and approximately 15,000 per square inch. The size and number'ofperforations cited are those measurable on microphotographs against astandard scale, but the figures given may be less than the actual numberbecause the smaller perforations will not pass enough light for thefilm.

Thestainless steel, prior to being subjected to the microperforatingprocess, may be sand blasted so as to produce a uniform mechanical etchresulting from blasting out by sand impact of a certain amount of steel.Where a smooth, light reflecting surface is not essential, the sandblasting allows a more uniform perforation pattern.

Forsome applications it is desirable to provide microperforations ofsubmicron size, particularly where the element is used for dialysis. Insuch cases the microperforated stainless steel produced according to theprocess above described may be impregnated with a microporous membraneproducing material. The choice of the impregnating material depends uponthe -applicationfor example, whether the final product is to be used forfiltering out micron or submicron size particles, whether it is toresist hydraulic flow and heat, yet be capable of the diffusion ofgasses or ions therethrough.

For most applications this involves the impregnation or the grainboundary microperforations with a liquid of a compound which when dryand solid can be leached out-that is to say, the liquid compound wouldhave a soluble component. Examples of suitable materials include resins,such as polystyrene, co-polymers of polyvinyl chloride andacrylonitrile, silicone lacquers, polyvinyl alcohol or its derivatives,with a leachable plasticizer and made insoluble by formaldehyde, ethyland methyl celluloses or cellulose esters, such as a solution ofcellulose acetate in acetone, rendered insoluble, gelatin renderedinsoluble with soluble chromates, etc. One type of water leachablemicroporous material is a 3% polyethylene with polyethylene oxide, thepolyethylene oxide being extracted with Water.

For applications requiring low hydraulic passage but high gas diffusion,I use stainless steel sheets of 1 mil thickness which have beenmicroperforated, washed and dried and subsequently impregnated with asilicone solution which when dry leaves a thin membrane of silicone on astrong base, thus permitting handling with ease for dialysisapplication. A suitable material is one derived from spraying a siliconesolution commercially known as Glaskote or Ucarsil R-l04 which contains4.7% silicone solids. The thin silicone membranes can be free from holesand still be permeable to gases as well as impermeable to low pressurehydraulic flow of salt solutions.

The porous stainless steel sheet may also be impregnated with aninorganic material such as an aqueous solution of sodium silicate inwhich the sheet is dipped. When dry the sheet is treated with the acidsolution, such as sulfur acid, which leaches out the sodium componentsand leaves a microporous insoluble silicate membrane. Thin fluorcarboncoatings may also be employed to form permeable membranes in the grainboundary pores.

The microperforated stainless steel sheets of this invention may be usedin many applications where a strong, oxidation-resistant filter ormicroporous member is desired. For example, as a gasoline, oil, blood orother liquid filter, an air filter in air conditioners, a lens or hoodin place of dark glasses for welding goggles, a base electrode for fuelcells, a microporous separator for electrochemical cells, gas generatorsand electrode reactive cells, etc.

I claim:

1. An electrically controlled process for producing microperforatedstainless steel which comprises connecting a sheet of stainless steelhaving a thickness between approximately .0005" and .005, containingintergranular iron chromium precipitates and containing as essentialelements a preponderance of iron and a lesser but significant amount ofchromium as the anode in an electrolytic cell having a non-polarizingelectrolyte and a cathode and discharging direct current through saidcell so as to anodically dissolve intergranular iron chromiumprecipitates in said sheet and to thereby produce a multiplicity oflight transmitting microperforations through said sheet, themicroperforations being primarily due to electrochemical action.

2. The process described in claim 1 characterized in that the stainlesssteel sheet has been fabricated from east and rolled alloy.

3. An electrically controlled process for producing thin microperforatedstainless steel sheet which comprises connecting a sheet of stainlesssteel having a thickness in the order of .0005 to .005" and containingintergranular iron chromium precipitates as the anode in an electrolyticcell having a non-polarizing electrolyte and a cathode and dischargingdirect current through said cell so as to produce a multiplicity oflight transmitting microperforations through said sheet at the grainboundaries by dissolution of iron chromium precipitates withoutsubstantial reduction in thickness of the sheet.

4. An electrically controlled process for producing a perforated bodywhich comprises connecting a sheet of iron chromium alloy having athickness in the order of .0005" to .005, containing intergranular ironchromium precipitates, and containing as essential elements apreponderance of iron and a lesser but significant amount of chromium asthe anode in an electrolytic cell containing a non-polarizingelectrolyte and a cathode and discharging substantially continuousunidirectional direct current through said cell for a time sufiicient toproduce a multiplicity of light transmitting microperforationssubstantially uniformly distributed through said sheet, saidperforations being primarily due to electrochemical action.

5. The process described in claim 4 characterized in that the cathodecomprises titanium.

6. The process described in claim 4 characterized in that afterstainless steel sheet has been microperforated, it is impregnated with amicroporous membrane producing material.

7. An electrically controlled process for producing microperforatedstainless steel which comprises connecting a sheet of stainless steelhaving a thickness between approximately .0005 and .005", containingintergranular iron chromium precipitates and containing as essentialelements a preponderance of iron and a lesser but significant amount ofchromium as the anode in an electrolytic cell containing an electrolytecomprising a nonpolarizing halogen containing solution and dischargingdirect current through said cell for a time suflicient to produce amultiplicity of light transmitting microperforations through said sheet,said perforations being primarily due to electrochemical action.

8. The process described in claim 7 characterized in that the halogencontaining electrolyte is selected from the group comprisinghydrochloric acid; hydrobromic acid; solutions of halogen salts whichwhen dissolved hydrolyze to an acidic electrolyte, solutions of alkaliand alkaline earth metal halogen salts, ammonium halogen salts andnatural brine solutions.

9. The process described in claim 8 characterized in that theconcentration of the hydrochloric acid does not exceed 7%.

10. An electrically controlled process for producing microperforatedstainless steel sheet suitable for fabrication into liquid and airfilters, diaphragms, separators and the like which comprises connectingstainless steel sheet consisting of a preponderance of iron andcontaining a minimum of approximately 11 /2 chromium having a thicknessbetween approximately .0005 and approximately .005" as the anode in anelectrolytic cell having a nonpolarizing electrolyte and a cathode anddischarging direct current through said cell for a time sufficient toproduce a substantially uniform pattern of a multiplicity of lighttransmitting microperforations through said sheet, said perforationsbeing primarily due to electrochemical action.

11. An electrically controlled process for the continuous rapidproduction of sheet filter material which comprises the provision of anelectrolytic cell having a cathode anci non-polarizing electrolyte inwhich a portion of a strip of stainless steel is immersed, said steelconsisting of a preponderance of iron and containing a minimum ofapproximately Il /2% chromium and having a thickness not substantiallyexceeding .005" and constituting the anode of said cell, dischargingdirect current through said cell while passing said sheet therethroughat a current density and for a time suflicient to produce a multiplicityof light transmitting microperforations through said sheet, saidperforations being primarily due to electrochemical action.

12. An electrically controlled process for the continuous rapidproduction of a microperforated stainless steel sheet which comprisesthe provision of an electrolytic cell having a non-polarizingelectrolyte and a cathode inert to said electrolyte, an elongated stripof stainless steel consisting of a preponderance of iron and containinga minimum of approximately 11 /2 chromium, a portion of said strip beingimmersed in the electrolyte and at anodic potential in respect thereto,said strip having a thickness in the order of .0005" to .005,discharging direct current through said cell, the strip being connectedas the anode, while passing said strip through the electrolyte at acurrent density, applied voltage and for a period of time sufficient toproduce a multiplicity of light transmitting microperforationssubstantially uniformly distributed through said sheet, saidperforations being primarily due to electrochemical action, thereaftercleaning said perforated sheet to eliminate retained electrolyte.

13. A rnicroperforated stainless steel product produced by the processof claim 1.

References Cited UNITED STATES PATENTS OTHER REFERENCES Belyakov et al.:Zavodskaya Laboratoriya, No. 10, 1961, pp. 11921194.

10 Miller et al.: Metal Progress, November 1949, pp.

Moneypenny: Stainless Iron and Steel, Chapman and Hall Ltd., London,1954, vol. 2, pp. 190, 191, 201, 204

and 205. 15

ROBERT K. MIHALEK, Primary Examiner.

JOHN H. MACK, Examiner.

1. AN ELECTRICALLY CONTROLLED PROCESS FOR PRODUCING MICROPERFORATEDSTAINLESS STEEL WHICH COMPRISES CONNECTING A SHEET OF STAINLESS STEELHAVING A THICKNESS BETWEEN APPROXIMATELY .0005" AND .005", CONTAININGINTERGRANULAR IRON CHROMIUM PRECIPITATES AND CONTAINING AS ESSENTIALELEMENTS A PREPONDERANCE OF IRON AND A LESSER BUT SIGNIFICANT AMOUNT OFCHROMIUM AS THE ANODE IN AN ELECTROLYTIC CELL HAVING A NON-POLARIZINGELECTROLYTE AND A CATHODE AND DISCHARGING DIRECT CURRENT THROUGH SAIDCELL SO AS TO ANODICALLY DISSOLVE INTERGRANULAR IRON CHROMIUMPRECIPITATES IN SAID SHEET AND TO THEREBY PRODUCE A MULTIPLICITY OFLIGHT TRANSMITTING MICROPERFORATIONS THROUGH SAID SHEET, THEMICROPERFORATIONS BEING PRIMARILY DUE TO ELECTROCHEMICAL ACTION.
 13. AMICROPERFORATED STAINLESS STEEL PRODUCT PRODUCED BY THE PROCESS OF CLAIM1.